Donald Trump has fired Secretary of State Rex Tillerson after a series of public rifts over policy on North Korea, Russia and Iran.

The US President has replaced his chief diplomat with hardline CIA Director Mike Pompeo.

Gina Haspel, Pompeo's deputy, is being nominated to replace him and if confirmed she would be the CIA's first female director.

Trump says the main reason for firing Tillerson was because they disagreed on the Iran nuclear deal.

Outgoing US Secretary of State Rex Tillerson did not speak to President Donald Trump before he was sacked (Image: AFP)

Iran agreed a deal with Barack Obama before Trump took office that saw it curb its nuclear programme in exchange for the easing of economic sanctions.

The president has regularly criticised the agreement as a bad deal and has repeatedly threatened to end it.

He spoke today of his desire to break the deal but noted that Tillerson "felt a little bit differently, so we were not really thinking the same".

Trump tweeted that Pompeo, "will do a fantastic job!" and thanked Tillerson for his service.

He said he made the decision to sack his secretary of state "by myself" and suggested Tillerson will be "much happier now" .

Pompeo is strongly opposed to the Iran nuclear deal and Trump said they have a "very, very similar thought process".

The biggest shakeup of Trump's Cabinet since he took office in January 2017 was announced by the president on Twitter as his administration prepares for an unprecedented meeting with the leader of North Korea .

Donald Trump answers questions after his firing of Secretary of State Rex Tillerson (Image: Getty Images North America)

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Tillerson's departure caps months of friction between the Republican president and the 65-year-old former Exxon Mobilchief executive.

The tensions peaked last fall amid reports Tillerson had called Trump a "moron" and considered resigning.

"We've been talking about this for a long time," Trump said on the White House lawn today.

"We got along actually quite well but we disagreed on things."

Senior State Department officials said Tillerson was unaware of why Trump pushed him out and that he had intended to stay in the job.

On Monday, Tillerson blamed Russia for the poisonings in England of a former Russian double agent and his daughter.

Earlier, White House press secretary Sarah Sanders had refrained from saying Moscow was responsible. Many saw this as another area of disagreement between Tillerson and Trump, who is mired in allegations about his close relationship to powerful Russians.

Sarah Huckabee Sanders did not blame Russia for the Salisbury poisoning (Image: REUTERS)

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A senior White House official said Trump asked Tillerson to step down on Friday but did not want to make it public while he was on a trip to Africa.

Trump's Twitter announcement came only a few hours after Tillerson landed in Washington after the trip, which had been cut short.

The official said Trump works well with Pompeo, a former Republican congressman from Kansas, and wanted him in place before the US president's planned talks with North Korean leader Kim Jong Un and trade negotiations.